Outside the old Medici fortress, on a tree-lined avenue near Santa Maria Novella station in Florence, a one-man offensive is being waged to force Italian prime minister Silvio Berlusconi to stand trial for corruption.

Mr Di Pietro needs 500,000 names by September 26, a goal well within reach. Day in and day out during the crippling heatwave, the independent senator has criss-crossed Italy by road and rail, armed only with personal zeal and a megaphone.

"Within two weeks, the objective will have been reached," he says, shaking hands with crowds of onlookers who mob a man they see as a hero of democracy. "I am going around Italy like a madman."

By the end of last week, his self-financed campaign had attracted 400,000 signatures. In markets and provincial fetes, he appealed to the crowds: "Please sign to ensure that the law is equal for all - even for the prime minister. Italians have a right to know now, not later, if their premier is guilty of corruption." Mr Di Pietro, a southern Italian, with poor, rural roots, has been welcomed by the very housewives who helped to vote Mr Berlusconi into power in 2001 but now respond to the senator.

In the early 1990s, when he was still a prosecutor, Mr Di Pietro was the scourge of Italy's corrupt ruling class, bringing many politicians to book.

Now his petition, which calls for a referendum next June, taps into the growing grass-roots anger over Mr Berlusconi's failure to keep election promises. Among the public there is widespread unease over the government's use of parliament to spare Mr Berlusconi - the subject of numerous investigations - from prosecution while in office.

Nonetheless, many supporters of Mr Di Pietro fear that referendum turnout may be too low for the result to be valid.

His campaign has been ignored both by the mainstream media, which is largely under the government's control, and the Centre-Left opposition. The opposition fears it may lose face if the referendum fails to win backing for a law change.

One Florence pensioner, Giuliano Cungi, speaks for many when he declares: "I've signed, because I refuse to let [the government] get away with it. But I'm afraid there may not be a quorum. The government has almost complete control of television. And the organisers have nothing."

However, they will fight on, says Luigi Sedita, 56, a hospital director who is on the local Di Pietro Party committee. "What distinguishes our movement, and also our petition, is that we are all ordinary people. None of us are politicians.

"We are involved in this campaign because the government is violating our constitution, which our fathers fought and died for in the resistance.

"The article says that the law is equal for all. But with this government, some have a better standing before the law than others."